1. Field of the Invention
This invention relates to a melt-moldable crystalline polyimide polymer and a process for producing the same.
2. Description of the Related Art
In general, since polyimides are organic high-molecular weight compounds having a high heat resistance and also superior electrical characteristics and mechanical characteristics, they have been broadly used as protecting materials, insulating materials, in the field of electronic equipments and also adhesives, films, structural units, etc. However, conventional polyimides have high melting points or glass transition points and these temperatures often exceed their thermal decomposition temperatures or are close thereto; hence it has generally been difficult to subject them to melt-molding. Thus, as current molding processes for polyimides, molding processes for thermosetting resins have been mostly employed such as processes for coating and baking a precursor solution of polyimides or a soluble polyimide solution or processes for subjecting powdery polyimide polymers to compression molding, etc. Accordingly, if a polyimide polymer to which extrusion molding or injection molding as molding processes for thermoplastic resins is applicable is obtained, it is possible to improve the productivity of polyimide molded products and also obtain various molded products therefrom; hence the economical effect will be great. Thus, meltable polyimide polymers have often been proposed. For example, U.S. Pat. No. 4558110 discloses a crystalline silicone imide copolymer. However, this copolymer has a m.p. of 140.degree.-210.degree. C. according to its Examples; hence it cannot help regarding the copolymer as having improved its process ability at the cost of its heat resistance specific of polyimides. Further, European patent application publication No. 200204 discloses a specific pyromellitic imide copolymer as a meltable polyimide copolymer, and also discloses that the copolymer is moldable into film by melt-press at 400.degree. C., and an effectiveness brought about thereby, but its melting point is not shown and whether the copolymer is crystalline or not is unclear. As described above, industrial need for high-temperature-meltable, preferably commercially melt-moldable polyimide polymers is great, but it is the present status that such polymers are still being groped for. Commercially melt-moldable polyimide polymers referred to herein mean those generally having a melting point lower by about 50.degree. C. than the thermal decomposition temperature thereof.